1. Field of the Invention
The present invention relates to computer buses, and more particularly to methods and apparatus for optimizing bus bandwidth.
2. Description of Related Art
Various computer bus structures and bus protocols are well known in the art. One such bus is the ISA bus. ISA stands for industry standard architecture, and is a bus specification originally developed for PC and XT type personal computers, which provide 8-bit slots for connection of platform resources, and for AT type personal computers, which provide 8-bit and 16-bit slots for connection of platform resources. Platform resources include user add-on cards, which are readily inserted and removed by the user. Originally found in computer systems based on the type 8088 and 80286 microprocessors, the ISA bus is also used in more advanced personal computers, including those based on the type 80386 and 80486 microprocessors. These microprocessors and the personal computer systems based on them are available from various manufacturers, including Intel Corporation of Santa Clara, Calif., and IBM Corporation of Armonk, N.Y.
The ISA bus is specified as a low performance bus, being limited to a data size of 16-bits, a memory address space of 16 megabytes ("MB"), and a bus clock speed of 8 megahertz ("MHz"). Newer system buses such as the EISA bus and various local buses such as the VESA bus and the PCI bus have been introduced. These newer buses are considerably faster than the ISA bus, and so are advantageous for communicating I/O signals between the system microprocessor and such high performance peripheral devices as high resolution graphical video displays and high speed cachable hard disk drives. However, because the PC, XT and AT type personal computer systems cannot be upgraded to the newer high performance buses without replacing their motherboards, which is a prohibitively expensive option, the ISA bus is likely to remain in widespread use. Moreover, ISA compatibility is increasingly being required for embedded systems.
Platforms using the ISA bus as their system bus cannot take full advantage of higher performance peripheral devices that can be made available in the form of user add-on cards which plug into the ISA bus. This situation is particularly unfortunate since the advent of multimedia. ISA bus platforms used for multimedia typically must be upgraded with high bandwidth peripheral devices such as high resolution graphics accelerators and multimedia add-on cards. The performance of these upgraded ISA bus platforms, compromised by their ISA bus, is often disappointing.